ZyXEL Communications ZYWALL USG 2000 Manual page 297

NAT Loopback Example
The NAT 1:1 Example on page 292
address of a LAN SMTP mail server to allow users to access the SMTP mail server
from the WAN. LAN users can also use an IP address to access the mail server.
However, you need to configure NAT loopback for LAN users to use a domain name
to access the server.
Figure 182 LAN Computer Queries the DNS Server
A LAN user computer at IP address 192.168.1.89 queries the domain name
(xxx.LAN-SMTP.com in this example) from a public DNS server and gets the SMTP
server's 1-1 NAT mapped public IP address of 1.1.1.1.
NAT Loopback Virtual Server
When a LAN user sends SMTP traffic to IP address 1.1.1.1, the traffic comes into
the ZyWALL through the LAN interface, thus it does not match the NAT 1:1
mapping's virtual server rule for SMTP traffic coming to IP 1.1.1.1 from WAN2. So
you must configure a similar virtual server rule for WAN2.
ZyWALL USG 2000 User's Guide
xxx.LAN-SMTP.com = ?
1.1.1.1
LAN
192.168.1.21
maps a public IP address to the private IP
DNS
xxx.LAN-SMTP.com = 1.1.1.1
192.168.1.89
Chapter 16 Virtual Servers
297